The radius and ulna together form the bony structure of the forearm. The two bones articulate with one another at both their proximal and distal ends. The distal radioulnar joint is a “shallow socket” ball joint. The ulna, a relatively straight bone, translates dorsal-palmarly to accept the modestly bowed radius. The distal end of the radius articulates in pronation and supination on the distal head of the ulna at the sigmoid notch or fossa. The sigmoid notch socket in most wrists is relatively flat and a number of ligaments support the distal ulna where it meets the distal end of the radius. The supporting ligaments include the triangular fibrocartilage (TFC), the extensor carpi ulnaris (ECU) subsheath, and the ulnar collateral ligament complex. The stabilizing elements of the triangular fibrocartilage, the extensor carpi ulnaris subsheath, and the ulnar collateral complex work in cooperation with the distal ulna to transfer compressive loads between the ulnar carpus and the distal ulna across the distal radioulnar joint.
Unfortunately, fractures of the distal radius and other injuries of the forearm commonly occur and may cause rotational instability. Following these injuries to the forearm, ligament disruption, ulnar styloid fractures, and fractures into the distal radioulnar joint commonly occur. Fracture or dislocation involving the distal radioulnar joint often results in a loss of forearm rotation related to either instability or incongruity between the sigmoid fossa of the distal radius and the head of the ulna. A variety of different fractures involving the distal radius can cause this condition including the Colles' fracture and the Galeazzi fractures.
When there is loss of stability of the distal radioulnar joint, subsequent weakness in grip and pinch as well as potential loss of forearm rotation occur. Instability can also be associated with an injury to the triangular fibrocartilage or to the ulnar styloid. When instability is present, a number of ligament reconstructive procedures have been devised to assist in treating the unstable distal ulna. Unfortunately, ligament reconstruction of the distal ulna often does not restore complete stability and joint replacement is often performed in an effort to stabilize the joint.
Sometimes, when the distal ulna is damaged, the preferred treatment is hemiarthroplasty. That is to resect the head of the ulna and replace it with an ulnar head prosthesis which then is in contact with the natural bone of the radius. The prosthesis then articulates with the sigmoid notch to restore functionality to the distal radioulnar joint. Unfortunately, long term articulation between the man made implant and the natural bone may accelerate wear of the bone and lead to arthritic or degenerative change. This may cause the patient pain and restriction of motion.
In other circumstances, both the head of the ulna and the distal radius may suffer injury, arthritic change or degenerative change simultaneously. Then, it would be desirable to replace the articular surfaces of both the head of the ulna and the sigmoid fossa.
In addition, in some cases, replacement of the ulnar head with a prosthesis does not fully restore stability to the distal radioulnar joint. At present, patients that have had an ulnar head resection with implantation of an ulnar head prosthesis who still suffer from instability have few options.
In light of the foregoing, the surgical arts would benefit from access to a prosthetic device that could be used to restore the function of a damaged sigmoid notch articular surface. The surgical arts would also benefit from the availability of a prosthetic combination to repair an injured or degenerated distal radioulnar joint.